fix(turbopack-css): treat css url resolve error as warning

[fireairforce]

把 css 里面的 url resolve 按照 optional 的 resolve 去处理:

https://github.com/utooland/next.js/blob/utoo/turbopack/crates/turbopack-core/src/resolve/mod.rs#L3390-L3392

这样 resolve 解析不到时，当作 warning 处理而不是 error 处理，这对很多存量业务 case 会很友好。

[gemini-code-assist]

Summary of Changes

Hello @fireairforce, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求旨在通过将 CSS URL 解析失败从硬错误降级为警告来增强 Turbopack 对现有项目的兼容性。这一更改通过将解析标记为“可选”来实现，确保即使某些资源无法解析，构建过程也能继续，同时仍能通知开发者潜在的问题。

Highlights

• CSS URL 解析行为变更: 将 CSS 中 URL 的解析行为从强制解析改为可选解析。
• 错误处理优化: 未能解析的 CSS URL 现在将作为警告而非错误处理，从而提高对现有项目的兼容性。

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[gemini-code-assist]

Code Review

此拉取请求将 CSS 中无法解析的 url() 引用从构建错误更改为警告。这是通过将资源解析标记为可选来实现的。虽然这对于处理包含无效 URL 的存量业务很有帮助，但这也可能导致在新项目中意外地忽略了损坏的资源路径。我添加了一条评论，建议将此行为设为可配置，以提供更大的灵活性。